Study on the Variability of Mixing and Salt Transportation along the Modaomen River in the Pearl River Estuary

The saltwater intrusion ranks as a key issue in the Modaomen River, which is one of the main channels in the Pearl River Estuary. It is recently considered as a major challenge of freshwater supply for several cities such as Zhuhai, Macao and Zhongshan. In order to investigate the dynamics of saltwater intrusion in the Modaomen River, measurement data including stratified velocity and salinity were obtained from eight hydrological vessels in a lower discharge season. According to these data, the salinity stratification parameter and bulk/layer Richardson number, which can be used to describe the state of mixing/stratification, are firstly calculated. Results show an obvious ebb-flood and neap-spring variation of mixing/stratification in the Modaomen River. Furthermore, the salt transport rate, accumulative salt flux and saltwater intrusion length are also calculated and compared. Results indicate that the salt transportation indicates an obvious surface-bottom difference during the neap tidal cycle and an obvious ebb-flood variation during the spring tidal cycle. The direction of salt transportation in deeper and up water is persistently landward and seaward respectively during the neap tidal cycle. The landward salt flux mainly occurs during the neap tidal cycle. Comprehensive analyses indicate that the saltwater intrusion in the Modaomen River is gradually intensified within the duration from neap to middle tide and then relaxed until the next neap tide. Thus the worst scenario of saltwater intrusion in the Modaomen River will usually occur in the before-spring middle tide during a semilunar tidal cycle.